I think that the best plan would be to strip components from the Ross and use the Maruishi frame as it seems to be a better fit for me. It is about 1/2" longer TT to BB and TT length.

Can anyone comment on the sizing of the bikes based on the attached picture? My mountain bike setup is provided for reference (first row), this riding position is pretty comfortable for me over my 14 mile (one-way) commute. Ross is second row, Maruishi is third row. I've never ridden a road bike before.

Are there any parts compatibility issues that I may run into between the two bikes?

Can anyone recommend a dependable, inexpensive rack/bag combination? I need to carry lunch, work clothes, and a laptop.

Sorry nobody's replied yet, so I'll try to post and maybe spur some other responses...

The one problem I see between the two bikes is that the Marushi doesn't have a rear derailleur hanger--it's integrated with the derailleur. I don't know how complicated it would be to separate the hanger from the derailleur. Another thought i had was that if you put fatter tires on either bike, fitting fenders might be a problem. It looks, from the images I could see, I think there are dropouts for racks, at least. do agree with you that the Marushi frame seems to be a better fit for you.

All that said, another option is putting thinner, slick tires on your mountain bike. :-)

The Ross is small for you from what it looks like in the picture. You look to be pretty close to 6ft. The Marushi looks to be about a 58-60CM. That would be about right for a 6ft tall person.
The Ross looks to be 54-56cm. The cannondale is a 22inch (56 cm MTB size smaller because of the higher BB but that again would be about the right size for someone 6ft).

Are you able to work on bikes? If so you should be able to come out good if not make a little profit if you have some patients.

I would start looking for a better frame or complete bike with cast drops with integrated RD mount in a 58-60cm range if I got the sizes of the bikes correct. The 600 Group is not bad stuff at all. I'd take the best wheels and best parts put them on a better frame. Clean up and flip the remaining parts.

The Ross complete ready to ride might bring as much as $150 on a flip. The Maruishi $100-125 depending on how it cleans up. It may clean up a lot better then you think. The Maruishi may also be a good bike to single speed and sell to a hipster if the derailers are in bad shape but the frame is relatively light. You can get a BMX free hub for $20-25.

Search my nick. I just cleaned up a 76 Varsity that was in really rough shape. Its amazing what you can do with the right stuff. I removed the rust with an electrolysis set up that was pretty cheap to build. The bike Turned out pretty nice and rode great. I had $35 in the bike including what I picked the bike up for at a local thrift and that included 2 new tires I cought on sale and a lot of elbow grease cleaning it up. I serviced all the bearings, derailers replaced some of the cables and sold the bike for $115. The Key is the labor, what I did to the bike would have easily been $150 in labor at most bike shops. I just put a few a couple hours in the bike a few nights in my free time.

If you are patient and watch local thrifts and CL like a hawk you will find something. May take a while.

In the mean time take a trip over to the Classic and Vintage forum and there is a LOT of good info on how you can clean up that Maruishi to make it a profitable flip bike to make a little money to get you into something fitting your needs better.

It's really difficult to say which position feels most comfortable for you. To my mind, both road bikes look like a better fit than the MTB. My strategy in this situation would be to ride both for a while and see which one "takes".

Rack: Nashbar has some Ascent racks which cost about $13 (and were even cheaper this weekend.). I bought one for my 80s fuji commuter and it is OK. Panniers: I have always commuted with DIY panniers made from used daypacks. I usually need to sling the bag on my back for a while to get to my desk. If that doesn't work, you can get some excellent deals on panniers on eBay. I also have Axiom Champlain panniers which I used for camping or day trips, but not for commuting as they don't have a shoulder strap.